As the size of devices scale down in modem Integrated Circuits (ICs), although transistor speeds improve, the number and the length of the interconnects increases. As a result, a performance bottleneck occurs in the interconnects, which negatively affects device performance. In addition, decreasing the size of the devices dramatically increases power dissipation stemming from both the larger number of interconnects and the degrading device power for a given performance. Three-dimensional integrated circuits (3D ICs) and germanium (Ge), which has a higher mobility than silicon (Si), are considered promising architecture-level and device-level solutions to alleviate large delay and power dissipation problems arising from interconnects and transistors, respectively. 3D integration allows for longer interconnects, which in turn improves both speed and power dissipation in the wires by reducing their capacitance. In addition, 3D-ICs serve as an ideal medium for integration of heterogeneous and disparate technologies such as, radiofrequency (RF), memory, logic, optical devices, thin film transistors (TFT), on-chip sensor, and on-chip micro-electromechanical systems (MEMS).
There are several types of 3D-ICs differentiated primarily by the degree of vertical interconnectivity. They include: 1) package level stacking and connections such as for cell-phone memories (limited to peripheral connections), 2) wafer-to-wafer or die to wafer bonding requiring through-via holes, and finally 3) monolithic 3D-ICs, which exhibit a bottom-up manufacturing of 3D layers, and potentially have the highest (gate-level) vertical interconnectivity. Although monolithic 3D-ICs possess maximum benefits, their fabrication is challenging. It requires low temperature processes preferably under 400° C. in order to preserve underlying layers. These layers consist of both an interconnect stack with fragile, porous low dielectric constant (K) materials and metals, as well as device layers, whose parameters, such as junction depths, have to be tightly controlled to enhance performance by limiting the thermal budget. Ge is an ideal substrate for 3-D ICs, having advantageous characteristics pertaining to both fabrication and performance. First, Ge has a lower melting point than Si, and therefore, it is conducive to low temperature processing. Second, it possesses a higher mobility than silicon, potentially leading to a better power-performance trade-off.
Illustrative, non-limiting embodiments of the present invention overcome the above disadvantages and other disadvantages not described above. Also, the present invention is not required to overcome the disadvantages described above, and an illustrative, non-limiting embodiment of the present invention may not overcome any of the problems described above.